On the same plateau is the entrance to the celebrated Catacombs of St. Calixtus. It is on the right-hand side of the road, about a mile and a quarter from the present gate, and near where stood the second milestone on the ancient Appian Way. A marble tablet over the door of a vineyard shaded with cypresses points it out to the visitor. The rock out of which this and all the Roman Catacombs were hewn seems as if created specially for the purpose. Recent geological observations have traced in the Campagna volcanic matter produced at different periods, when the entire area of Rome and its vicinity was the seat of active plutonic agency. This material is of varying degrees of hardness. The lowest and oldest is so firm and compact that it still furnishes, as it used to do, materials for building; the foundations of the city, the wall of Romulus, and the massive blocks on which the Capitol rests, being formed of this substance. Over this a later stratum was deposited called _tufa granolare_, consisting of a similar mechanical conglomerate of scoriae, ashes, and other volcanic products, but more porous and friable in texture. It is in this last formation, which is so soft that it can be easily hollowed out, and yet so solid that it does not crumble, that the Catacombs are invariably found. There is something that appeals strongly to the imagination in the fact that the early Christians should have formed the homes of their dead and the haunts of their faith in the deposit of the terrible volcano and the stormy sea! The outbursts of the Alban volcanoes were correlated in God's scheme of providence with the outbursts of human fury long ages afterwards; and the one was prepared as a means of defence from the other, by Him who maketh His ministers a flaming fire.